CLSA-2026-1789554546 libxml2: Fix of 2 CVEs

CVE-2026-86143: guard the sizet buffer length against INTMAX before it is narrowed to the int passed to writecallback in xmlOutputBufferWrite, xmlOutputBufferWriteEscape and xmlOutputBufferFlush xmlIO.c, so a negative length can no longer reach an output callback - CVE-2026-86144: propagate the...